Maybe some basic stuff like:
Spells:
Healing book:
Lvl 1: +2 heal
Lvl 2: 2 x +2 heal
Lvl 3: 3 x +3 heal (or +4?)
Units:
Reviver:
revives a killed unit in all lanes once per game, counts as summoned
Lvl 1: ceil(stats/4), cannot be rerevived
Lvl 2: ceil(stats/2), cannot be rerevived
Lvl 3: full stats, can be rerevived (or revives with blessing)
Grave digger:
digs up a skeleton when a unit dies, doesn't work on summons
Lvl 1: 2 times, 1/1 skeleton
Lvl 2: 3 times, 2/2 skeleton
Lvl 3: 4 times, 4/3 skeleton
Jinx:
on death curses the first enemy
Lvl 1: 1 dmg x 1 turns (just to debuff)
Lvl 2: 2 dmg x 2 turns
Lvl 3: 3 dmg x 3 turns
Leech:
absorbs the power of the enemy on hit
Lvl 1: steals 1/1
Lvl 2: steals 2/1
Lvl 3: steals 4/2
Vampire:
heals for a percentage of the damage dealt
Lvl 1: heals floor(damage dealt/4)
Lvl 2: heals floor(damage dealt/2)
Lvl 3: heals full damage dealt
Pusher:
pushes back the enemy on hit
Lvl 1: 1 square 1 turn
Lvl 2: 1 square 2 turns
Lvl 3: 2 squares 2 turns
Items:
Spiked Shield:
absorb 1 damage and also returns 1 damage